The present invention relates to a method for generating an X-ray image and an apparatus thereof for correcting a blur of a fluorographic image or radiographic image.
A fluorographic image or radiographic image obtained by an ordinary fluorographic device includes a blur generally and to obtain a more desirable fluorographic image or radiographic image, a correction process of removing this blur is necessary. Firstly, the cause ofblur occurrence will be explained.
In radiography or fluorography, the intensity distribution of X-ray passing through a subject to be inspected is converted to an optical image by an X-ray image sensor and the optical image is recorded on a film or the optical image is electrically read by a TV camera and displayed and recorded digitally.
For example, in radiography, there are direct radiography using an X-ray intensifying screen and an X-ray film and indirect radiography using an optical system comprising an X-ray phosphor plate, lens, and mirror and a film. In real-time fluorography, there is a method for forming an optical image of the intensity distribution of X-ray converted by an X-ray phosphor plate and an X-ray image intensifier on the imaging surface of an imaging tube using an optical system such as a lens and electrically reading it.
FIG. 2 shows a typical X-ray detection system of the prior art using an X-ray image intensifier (hereinafter abbreviated to XII). Among X-rays irradiated from an X-ray tube 101, the intensity distribution of X-rays on the input surface of an XII 103 by direct X-ray 111 passing through a subject 110 and an anti-scatter grid 102 is converted to an optical image on an output phosphor screen 104 of the XII 103 by scanning of an electron beam 109 and this optical image is formed on an imaging device 106 using a lens and a mirror 105 and electrically read.
On the output phosphor screen 104, a diffusion light 108 which is called a veiling glare caused by a light diffusion phenomenon is generated in addition to a direct light 107 and an image by the original X-ray intensity distribution is blurred. Generally in a method that the X-ray intensity distribution (X-ray image) is converted to an optical image and then the optical image is optically measured, a phenomenon that in a phosphor medium generating an optical image, the original X-ray image is blurred by the light diffusion phenomenon cannot be avoided. The blur due to veiling glare affects strongly as the reduction rate of an X-ray image in a phosphor medium increases. The veiling glare 108 in the XII 103 causes a reduction of the image contrast.
Another cause of blur in addition to the veiling glare 108 is a scattered X-ray 112 by the subject 110. This scattered X-ray 112 travels in a direction different from that of an X-ray beam (hereinafter abbreviated to "direct X-ray") 111 emitted from the X-ray tube. Scattering of X-rays due to a subject is always generated, so that when a scattered X-ray is detected by an X-ray image sensor, the X-ray intensity distribution on the surface of the X-ray image sensor is blurred. Generally, the anti-scatter grid 102 is arranged on the front of an X-ray image detector so as to shield a scattered X-ray. However, all scattered X-rays entering the grid cannot be shielded and a measured image includes a blur due to the scattered X-ray. To the image on the surface of the X-ray image detector including the blur due to a scattered X-ray, a blur due to the veiling glare 108 in the aforementioned phosphor medium is added furthermore.
As explained above, in the method for converting an X-ray image to an optical image and measuring it optically, the blur on the surface of X-ray image sensor caused by a scattered X-ray by a subject and the blur due to the veiling glare in the phosphor medium reduce the image quality.
The related prior art for correcting a blur due to veiling glare and a blur due to scattered X-ray will be explained hereunder.
(1) An art for correcting a blur due to veiling glare in an XII by the direct deconvolution method is disclosed in the reference by Seibert and others (Medical Physics, Vol. 12, p. 281 to 288 (1985), Literature 1). An art for correcting a blur due to veiling glare and then correcting a blur due to scattered X-ray by the direct deconvolution method is disclosed in another reference by Seibert and others (Medical Physics, Vol. 15, p. 567 to 575 (1988), Literature 2). A degradation model and a correction process by the aforementioned correction methods of the prior arts are shown in FIGS. 3A and 3B. In the degradation model, the image blurring process due to scattered X-ray is expressed by a convolution 203 of a point spread function including scattered X-ray component 202 and an ideal image (virtual image excluding a blur due to scattered X-ray and a blur due to veiling glare) 201. Furthermore, the blurring process due to veiling glare is expressed by a convolution 206 of a point spread function (abbreviated to PSF (point spread function))including veiling glare component 205 and a blurred image by scattered X-ray 204.
The outline of the correction process according to the degradation model is described below. By a deconvolution 209 which is introduced from an exponential function expressing the veiling glare component of the point spread function and uses a deconvolution filter for veiling glare 208, a veiling glare corrected image 210 correctted a blur due to veiling glare is generated. Furthermore, by a deconvolution 212 which is introduced from a normal distribution function expressing the scattered X-ray component of the point spread function and uses a deconvolution filter for scattered X-ray 211, a corrected image 213 corrected blurs due to veiling glare and scattered X-ray is generated. The deconvolution is processed by performing the two-dimensional inverse Fourier transformation for the multiplication result of the two-dimensional Fourier transformation result of an image to be processed and the deconvolution filter. The deconvolution filter for veiling glare 208 includes the intensity ratio of veiling glare to direct light hereinafter abbreviated to "veiling glare intensity ratio") as a parameter. The deconvolution filter for scattered X-ray 211 includes the intensity ratio of scattered X-ray to direct X-ray (hereinafter abbreviated to "scattered X-ray intensity ratio") as a parameter. The veiling glare intensity ratio of the XII is obtained beforehand. The scattered X-ray intensity ratio is estimated by searching a look-up table prepared for each measurement condition beforehand from the value of measured image. The look-up table is generated beforehand for each X-ray tube voltage, thickness of subject, anti-scatter grid, diameter of field of view, and geometry for measurement.
(2) A prior art for correcting by the blurred image formation method is disclosed in the reference by Molloi and others (Medical Physics, Vol. 15, p. 289 to 297 (1988), Literature 3) and in the reference by Honda and others (Medical Physics, Vol. 20, p.59 to 69(1993), Literature 4). A degradation model and a correction process by the correction method using the aforementioned image formation method of the prior art are shown in FIGS. 4A and 4B. In the degradation model, the blurring process due to scattered X-ray and the blurring process due to veiling glare are integrated.
A product (hereinafter abbreviated to "veiling glare and scattered X-ray intensity distribution function") 304 of a sum intensity ratio of veiling glare and scattered X-ray component to direct X-ray component (hereinafter abbreviated to "veiling glare and scatter sum ratio") 302 and a point spread function of sum of scattered X-ray and veiling glare (hereinafter abbreviated to "point spread function of scattered X-ray and veiling glare") 303 is generated, and a blurred image 306 comprising a component in which veiling glare and scattered X-ray are integrated(hereinafter abbreviated to "image of scattered X-ray and veiling glare component") is generated from a convolution 305 of the product 304 and an ideal image 301, and a measured image 308 is expressed by addition 307 of the image of scattered X-ray and veiling glare component 306 and the ideal image 301.
The outline of the correction process according to the degradation model is described below. An image of scattered X-ray and veiling glare component 310 is generated from a deconvolution 309 of the product 304 and the measured image 308. The corrected image 302 is generated by subtracting 311 the image of scattered X-ray veiling glare component 310 from the measured image 308. With respect to the deconvolution 309, the convolution method is disclosed in the reference by Molloi and others and the Fourier transformation method is disclosed in the reference by Honda and others. In the deconvolution 309 in the convolution method, a two-dimensional convolution of the veiling glare and scattered X-ray intensity distribution function 304 and the measured image 308 is performed
The deconvolution 309 in the Fourier transformation method is performed as indicated below. As a function of the product of the two-dimensional Fourier transformation of each of the veiling glare and scatter sum ratio 302 and the point spread function thereof 303, a deconvolution filter in the spatial frequency space is obtained. The result of multiplication of the two-dimensional Fourier transformation of the measured image 308 by the deconvolution filter is subjected to the two-dimensional inverse Fourier transformation.
The veiling glare and scatter sum ratio is a function of the value of measured image, X-ray tube voltage for measurement, thickness of subject, anti-scatter grid, diameter of field of view, and distance between subject and grid (air gap and others).
As a veiling glare and scatter sum ratio decision method, a method for obtaining the ratio by searching a look-up table prepared for each measurement condition beforehand from the value of measured image is disclosed in the aforementioned reference by Molloi and others.
As another decision method, a method for calculating the ratio by searching the table and processing from the maximum value of measured image and the measurement condition is disclosed in the aforementioned reference by Honda and others and another reference by Honda and others (Medical Physics, Vol. 18, p. 219 to 226 (1991),Literature 5).
As explained above, the blur usually occurs in an X-ray fluoroscopic image and an X-ray radiographic image directly obtained by using a conventional radiographic apparatus. In order to obtain inherent X-ray fluoroscopic image and X-ray radiographic image, a correction process for eliminating the blur is necessary.
In a method of transforming an X-ray image to an optical image and measuring the optical image, the blur occurring on an X-ray image sensor face by a scattered X-ray inside the subject and a blur caused by a veiling glare in an optical image formation medium are added to an inherent X-ray image, thereby deteriorating the picture quality.
Especially, in an apparatus such as a cone-beam CT apparatus for imaging a three-dimensional distribution of the inside the subject on the basis of a plurality of X-ray images of the subject acquired from many directions around the subject, when blurs are added, in a region where the value of a pixel, that is, the pixel value should be the same, a deviation occurs in the pixel values in each image. Consequently, there are problems such that the picture quality of a three-dimensional image of the inside the subject reconstructed from the X-ray images also deteriorates and the uniformity of the CT number is lowered so that accurate diagnosis cannot be performed. As a method of imaging a three-dimensional distribution of the inside the subject, for example, there is a data correction processing method described in "JAMIT Frontier '95", Medical image engineering research, lecture paper collection, pp. 23-28 (1995)(Literature 6).
There are some known methods of correcting the blur caused by the veiling glare and the blur caused by the scattered X-ray. For example, there is a radiographic apparatus using a method of simultaneously correcting the veiling glare and the scattered X-ray described in "Medical Physics", Vol. 20, pp. 59-69 (1993) (Literature 7). As another method, there is a radiographic apparatus using a method of separately correcting the veiling glare and the scattered X-ray described in "Special Issue of Japan Medial Image Engineering Association Meeting", Vol. 14, No. 4, pp. 383-384 (Literature 8).